This invention relates to a backwashable filter, particularly for liquids, comprising a hollow-cylindrical filter element which can be rotated about its axis, through which a radial flow passes and which separates a filtrate chamber from a dirty-liquid chamber inside a filter housing, the filter being provided with a drive mechanism to rotate the cylindrical filter element about its axis and with a stationary backwashing device comprising a dirt removal channel arranged parallel to the axis of the filter element and leading to a dirt discharge conduit.
Mueller et al., German Patent No. DE-PS 3,209,216 discloses a liquid filter which is backwashable in sections and in which a hollow-cylindrical filter element is provided. This filter element separates a filtrate chamber inside a filter housing from a dirty-liquid chamber. The liquid filter also comprises a dirt removal channel which is arranged parallel to the axis of the filter element and has a U-shaped cross-section.
In order to clean the surface of the liquid filter, the filter is rotated about its axis and the dirt removal channel opened up so that, due to the pressure which prevails in the interior of the hollow-cylindrical filter element, liquid flows through the filter material arranged on the surface of the filter element. Dirt particles adhering to the filter material are carried along by the flow and are discharged through a dirt removal channel.
Normally, the size of the particles contained in the liquid to be cleaned cannot be precisely defined. Thus, under certain circumstances, particles may also be deposited which have diameters larger than the gap width of the filter. The cleaning away of such particles by means of the known backwashing device is made more difficult by the fact that, on the one hand, this backwashing device has a U-shaped cross-section which is as closed as possible and is open only to the filter area, but that, on the other hand, larger particles on the circumference of the filter element must also enter the dirt removal channel. As a result, there is a considerable gap at the front edge of the dirt removal groove through which liquid from the dirty-liquid chamber enters the dirt removal channel. This significantly reduces the backwashing effect on the filter element.
Boemer et al., Published European Patent Application No. EP 49,746 discloses an edge filter for lubricating oil which is provided with a cleaning element. The cleaning element has a rinsing duct and, on the one hand, has openings which face the filter surface for backwashing the edge filter as well as an inlet throat for precleaning the filter surface. By the use of this cleaning element, coarse dirt is to be removed by means of the inlet throat and the dirt deposited in the filter gaps is to be removed via the backwashing openings.
A disadvantage of this system is that, due to the arrangement of the inlet throat for coarse dirt, the backwashing effect is severely diminished.
Although the cleaning element represents a compact combination, it has the disadvantage that dirt of varying composition cannot be removed effectively. Thus, on the one hand, the inlet throat must be kept as small as possible in order to not unnecessarily influence the backwashing effect. But, on the other hand, a smaller inlet throat has the disadvantage that coarse dirt will deposit in front of the cleaning gap and cannot be carried away.
The low backwashing effect of the cleaning element is further reduced by the fact that a very wide circumferential area of the filter is backwashed and the velocity of the backwashing flow is therefore greatly reduced.